Radiometer and water indicating method

ABSTRACT

A radiometer and a method for indicating water content and water content change caused by water and/or liquid water are disclosed. According to the method, changes of noise temperature values are observed by receiving microwave electromagnetic radiation at a medium frequency. This medium frequency is considerably below the lowest absorption resonance (approximately 22 GHz) of water molecules. At least one noise temperature value is set as reference value, and noise temperature changes are compared with the reference level.

FIELD OF THE INVENTION

The objective of the invention is a radiometer and a method forindicating water content, for example, water content change caused bywater vapor and/or liquid water.

The radiometer is a sensitive receiver for small power levelmeasurement. The radiometer deviates from a conventional receiver in twoways. Its input signal is phase-incoherent and broadband, which meansthat it has noise character; in the receivers generally phase-coherentand almost monochromatic. Secondly, conventional receivers require tooperate as signal and noise ratio a considerably higher value than one.The actual radiometric `signal` effect is generally much lower than thereceiver's own noise level. The radiometer measures the receivingequipment's system noise temperature, which consists of the antennanoise temperature and the receiver noise temperature. The antenna noisetemperature is formed of the noise effect coming via the main beam andthe side lobes according to diagram 1.6.

BACKGROUND OF THE INVENTION

The essential atmospheric agents for microwave radiometry are oxygen andwater vapor. The atmospheric oxygen and water vapor emit on a cloudlesssky thermal noise and provide the so called clear sky radiometricbrightness temperature T_(sky). Inspected from the ground, theatmospheric brightness temperature at clear weather is a function ofboth frequency and elevation. The frequency dependence is due to theresonant absorption/emission spectrum of water and oxygen molecules. Dueto the atmospheric pressure the spectral lines are spread on a broaderfrequency range. The lowest spectral line of the water moleculeabsorption/emission resonance is at approx. 22 GHz frequency (FIG. 6a).The elevation angle dependence of the clear sky brightness temperatureresults from geometry. The transmission path length of the layer formedon the ground by the atmosphere is considerably shorter in the zenithdirection than closer to the horizon (FIG. 6b). The radiometricbrightness temperature of the atmosphere is to a certain extentdependent on the amount of effective agent in the radiometer beam, atclear sky on the so called effective path length of the inspectiondirection. The clear sky brightness temperature in zenith direction isthus considerably lower than close to the horizon (FIG. 6a).

Water is present in the atmosphere in water vapor and liquid form and asice in clouds and rains. Atmospheric water content changes: airhumidity, clouds and rain occur in the microwave region as changes inthe sky brightness temperature.

Atmospheric property observations with a radiometric scanner and rainindication with a rain detector are presented as examples of the fieldsof embodiment of the method and device according to the invention.

Atmospheric and ground properties have been measured by microwaveradiometers both from satellites (weather and remote sensing satellites)and from the ground.

Atmospheric microwave radiometric measurements from the ground have beenutilized for example, in meteorological applications, in measurementsrelating to interferometric and electromagnetic wave propagationstudies, e.g.:

i) the U.S. Pat. No. 4,873,481

ii) Measurement of atmospheric water vapor with microwave radiometry; S.Elgered et al./Chalmers University of Technology, Sweden,

iii) Utilization of the radiometry method in a satellite connectionpropagation study; T. Kokkila, thesis for diploma, University ofTechnology 1988,

iv) Correction of satellite beacon propagation data using radiometermeasurements; Stutzman, Haidara, Reklus, IEEE Proceedings.-Microwaves.Antennas. Propagation, Vol 141 No.1 Feb 1994,

v) Use of radiometers in atmospheric attenuation measurements Allnut,Pratt, Stutzman, Snider IEEE Proceedings.- Microwaves. Antennas.Propagation, Vol 141 No.5 Oct 1994.

The measuring device used in the above references are radiometers ofDicke-type (i, ii, iii) and a total output radiometer (iv, v). Theradiometers are multichannel or connected to a measuring systemutilizing radiometric measurements was to determine atmosphericproperties by brightness temperature absolute values. The measuring ofthe brightness temperature absolute values requires stabilization of theradiometer amplification, measuring result calibration, accurateknowledge of the antenna side lobe properties and ambient radiationproperties. The stabilization of the radiometer amplification is basedon the construction principle (Dicke) or on a regular reference loadmeasurement (total output radiometry). Calibration of the measuringresults can be implemented by known objects giving hot/cold-brightnesstemperatures or by artificial loads, for example, by placing before theradiometer antenna input a piece of space cloth having the ambienttemperature and alternately a piece of space cloth cooled with e.g.liquid nitrogen. The side lobe properties of the antenna can beestimated by measuring the antenna beam figure at used frequencies. Theambient radiation properties can be estimated by known radiationproperties of the ground. Based on this and the above mentionedreferences, the determination of the atmospheric brightness temperatureabsolute values requires complicated equipment, `scientific instruments`as well as difficult and expensive measurement systems.

The atmospheric weather phenomenons can be also observed by a radar. Theeffect transmitted by the radar scatters from the water drops thusrevealing possible water containing objects. Use of the radar requires atransmitter/receiver equipment. The curvature of the ground causes ashadow region, which restricts the operation range of the radar. Thecurrently used rain indicating detectors are based on the observation ofsome electric property change in the detector component caused by therain (rain drops or snow flakes), for example, the capacitance orbreakdown voltage. The mechanical constructions of these detectors aredue to their operation principle open and therefore sensitive tomalfunctions caused by fouling and require regular service.

One rain detector application area is the automatic control of, forexample the heating systems of satellite earth station antennareflectors. The wet snow gathered on the earth station antenna surfacesattenuates the signal and turns the antenna beam away from the satellitedirection thus reducing the capacity of the antenna. The rain detectorsfunction in connection with outdoor temperature detectors, controllingthe reflector heating into function when it rains at the temperaturearea of approx. -5°-+5° C. At higher temperatures the reflector snowmelts by itself and at lower temperatures there is the risk that themelted snow freezes to the antenna constructions. Dry frozen snow hasalso a smaller effect than wet snow.

In addition to the above mentioned mechanical constructiondisadvantages, the present rain detectors in the satellite earth stationheating control system have the deficiency that they in certainconditions do not observe the problems caused by snow gathered on theantenna surface. Frozen snowfall (for example, 10° C.) and its gatheringon the reflector surface does not switch on the heating control system.The warming of the gathered frozen snow when the weather becomes warmeror the sun has warmed the reflector surface increases the liquid watercontent of the snow. The melting of the gathered frozen snow on theantenna surface might cause long breaks or quality deteriorations in thetelecommunication for several days after the snowing.

SUMMARY OF THE INVENTION

The method and device according to the invention provide observations ofwater content changes with a simple microwave total output radiometer,the medium frequency of which is considerably below the lowestabsorption resonance frequency (22,3 GHz) of the water molecule. Theinvention enables the observation of, for example the presence ofatmospheric clouds and rain cells and provides information about theirproperties in a new way.

The method, equipment and applications presented are based onutilization of the relative and/or differential measurement results ofthe antenna noise temperatures. The measurement method createsconditions in which the phenomenon observed causes the brightnesstemperature change seen by the radiometer. The radiometer antenna beamis directed in the direction giving the basic brightness temperaturelevel, for example radiometricly to the "cold" sky. The appearance ofwater in the antenna beam causes the radiometer to see the brightnesstemperature change. The antenna beam can be directed straight in adirection giving the basic level or via the conducing surface. The basiclevel object can also be, for example a frozen space cloth or aradiometric hot piece.

The invention provides a new solution which eliminates the disadvantagesof the above described atmospheric observation methods and raindetectors. The invention is characterized in what is presented in theclaims below.

The devices according to the invention monitor the radiometer antennanoise temperature changes. The side lobe or ambient noise radiationproperties need not to be known, as these are not required for themeasurement results. The measuring principle is relative and/ordifferential forming the reference level of some of the measuringresults. The reference level floats, which means that it conforms to theslow changes of the noise radiation properties. At the frequency bandused, water causes a noticeable change in the atmospheric brightnesstemperature, but no particular disadvantages when gathered in theequipment constructions.

Separate reference loads as stabilization references or absolute valuecalibrations are not used, thus avoiding the complicated and expensiveconstruction of radiometers of Dicke-type as well as the inconvenientperiodic stabilization of the total output radiometers and the absolutevalue calibration of both radiometer-types.

The simple radiometer used in the method can to its main parts beassembled of conventional components. The device parameters have beenchosen to provide a high radiometric sensitivity (smallest expressiblenoise temperature difference) and a big dynamic range for waterindication at the applicable frequency band. The method according to theinvention provides new means of obtaining information about theproperties of atmosphere, clouds and rain cells.

According to one form of embodiment the solution of the invention can beused at satellite earth stations to improve the heating system controlof the antenna reflectors, thus also improving the quality of thesatellite connection.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described with reference to the enclosed drawings, inwhich

FIG. 1 shows a basic diagram of the clear sky and ground brightnesstemperature and the antenna noise temperature as a function of elevationat a frequency considerably below 22 GHz,

FIG. 2a presents a block diagram of the total output radiometerprinciple,

FIG. 2b presents a block diagram of the Dicke-radiometer principle,

FIG. 3a presents an equipment for the radiometric scanner application,

FIG. 3b shows the radiometric scanner operating principle,

FIG. 3c shows a functional block diagram of the radiometric scanner,

FIG. 4 shows a plane diagram output of the radiometric scanner,

FIG. 5a shows the principle of the radiometric scanner network,

FIG. 5b shows the principle of the radiometric scanner network combinedoutput,

FIG. 6a presents the clear sky brightness temperature as a function ofthe frequency at different elevation angles,

FIG. 6b presents the length of the atmospheric penetration path inzenith and close to the horizon,

FIG. 7 shows a snow-/rain detector application of a satellite earthstation heating system.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 presents the clear sky and ground brightness temperature and theantenna noise temperature as a function of elevation at a frequencyconsiderably below 22 GHz. The clear sky brightness temperature risesfrom a few Kelvin in zenith (elevation 90°) to over one hundred Kelvinon the horizon (0°). The brightness temperature of the ground is about290 K. When the antenna is directed to zenith, the antenna noisetemperature is formed of the visible brightness temperature from themain beam direction and also partly of the higher brightnesstemperatures visible from the side lobe and the back lobe directions.The noise temperature of the antenna becomes thus higher than the skybrightness temperature visible in the main beam direction. The antennanoise temperature at small elevation angles rises quickly when the morepowerful side lobes and the main beam are directed towards the ground.If there is a cloud or a rain cell in the main beam direction, theantenna noise temperature rises compared to the clear sky antenna noisetemperature at this elevation. In FIG. 1 the curve T.sub.(ClearSky)illustrates the clear sky brightness temperature in relation to theelevation angle. T_(Ant) illustrates the antenna noise temperature.T_(Ant)(ClearSky) illustrates the noise temperature at clear sky.T_(Ant)(Cloud/Rain) illustrates the antenna noise temperature when theradiometer receives thermal noise from a cloud or rain cell. Asreference level is presented in dashed line the curve portionillustrating the antenna noise temperature caused by the clear sky.T.sub.(Earth) illustrates the brightness temperature of the ground.

FIG. 2 shows a principle block diagram of the total output radiometer.The noise effect RF coming via the antenna 1 is limited in the bandfilter 2 and transmitted via the amplifier 3 to the detector 4.Essentially the same total output radiometric function is alternativelyobtained with the superheterodyne-principle. In this the output of thepre-amplifier 3 is led to the mixer 3a, from the output of which isband-filtered with 3b an intermediate frequency signal, which is furtheramplified in the intermediate frequency amplifier 3c and transmitted tothe detector 4. The intermediate frequency is formed of the differencebetween the RF-frequency and the local oscillator 3d frequency. Thesignal received after the detector is low pass filtered in 5. The outputprovides a voltage V_(out), which is proportional to the noise effectcoming to the detector, which voltage contains a direct voltagecomponent V_(DC) and an alternating voltage V_(AC).

    V.sub.out =V.sub.DC +V.sub.AC                              (1.1)

In the ideal case, the radiometer sensitivity, the smallest expressiblenoise temperature difference ΔT is directly proportional to the systemnoise temperature T_(SYS) and inversely proportional to the square rootof the radiometer bandwidth B and the low pass filter integration timeτ: ##EQU1##

The system noise temperature is formed of the antenna noise T_(A) whichis the antenna noise temperature T_(Ant), and the radiometric receivernoise T_(R).

    T.sub.SYS =T.sub.A +T.sub.R.                               (1.3)

The amplification variation ΔG_(S) occurring in the radiometers inpractice weakens the radiometric sensitivity: ##EQU2## in which G_(s) isthe radiometric total amplification.

The sensitivity of the total output radiometer is defined according toformula 1.4. In Dicke-radiometers (FIG. 2b) the sensitivity weakeningcaused by amplification variations is prevented by calibrating theradiometer with the switch 10 to a reference load 11 caused by a knownnoise temperature. The operation of the switch 10 and the phase detector7 are controlled by the oscillator 9, having typically a frequency inthe range of hundreds of hertz.

The sensitivity achieved with the Dicke-radiometer (lowest measurablenoise temperature difference) is: ##EQU3## i.e., two times inferior tothe one of the total output radiometer.

The microwave radiometer is often furnished either with a horn orreflector antenna. The (virtual) radiometric brightness temperatureT_(AP) visible from the ambience in the radiometer antenna isdirection-dependent T_(AP) (θ, Θ). The noise temperature T_(A) of theradiometer antenna is formed of the brightness temperature T_(AP) ((θ,Θ) weighted with the antenna beam figure

    P.sub.N (θ, Θ). ##EQU4##

One application of the method and the total output radiometer adaptedthereto is the radiometric scanner (FIG. 3a). It comprises the antenna1, the radiometric receiver 12, the antenna elevation drive 13, theelevation angle detector 14, the antenna azimuth drive 15, the azimuthangle detector 16, the cabling 17 and the control/data processing unit18 with the display unit. The scanner antenna can be placed inside theradome 19. The radiometric scanner observes the atmosphere from theground (ground based radiometer) and is placed so that the visibility tothe horizon is as unobstructed as possible.

The scanner (FIG. 3b) antenna 1 is turned in the elevation and azimuthangles so that the main beam K scans the elevation angle below thehorizon H towards zenith Z. The path is illustrated by dotted lines.When the elevation scanning is finished, the azimuth angle is turned anda new elevation scanning is started, thus covering the holeelevation-azimuth-angle-area to be scanned. The solid curves 20illustrate the curves of the same brightness temperature. In the cloud Pand the rain S the curves indicate their higher brightness temperaturesin relation to the rest of the ambience.

FIG. 3c presents an operational block diagram of the radiometricscanner. The radiometric receiver 12 obtains a received noise effectfrom the antenna 1. The receiver output is connected to the dataprocessing/processing/display unit 18, which also controls the azimuthdrive 15 and the elevation drive 13 through the antenna control unit 30.The data processing/processing/display unit 18 receives the antennaposition data from the azimuth angle detector 16 and the elevation angledetector 14 through the antenna control unit.

During operation the voltage V_(out) value obtained from the radiometeroutput is recorded as a function of the azimuth and elevation angle andthe time V_(out) (Az,El,t). The voltage is proportional to the systemnoise temperature T_(SYS) (Az,El,t). When the radiometric receiver's ownnoise temperature T_(R) is known, the radiometric antenna noisetemperature T_(A) can be calculated

    T.sub.A (Az,El,t)=T.sub.SYS (Az,El,t)-T.sub.R.

At clear sky in the elevation scanning area, the radiometric antennanoise temperature changes as a function of the elevation angle startingat zenith from smaller to bigger T_(A) ClearSky (El). When the antennamain beam is below the horizon, the antenna brightness temperature isclose to the physical temperature of the ground T_(A) Ground. Themagnitude of the antenna noise temperature caused by the clear sky inzenith T_(A) Zenith and to what level it rises when approaching thehorizon, depend on the medium frequency of the radiometric receiver andon the antenna side lobe properties. The antenna noise temperature isformed of the combined effects of ambience and the antenna properties(formula 1.6). The antenna noise temperature of a radiometer monitoringthe atmosphere from the ground in clear sky conditions is due to horizonlevel and variations in ground radiation properties dependent on theantenna elevation angle and also on the azimuth angle.

The antenna noise temperature graph T_(A) ClearSky (Az,El) obtained inclear sky conditions forms the basic reference of the scanning resultanalysis.

If, during elevation scanning the antenna beam hits a water containingobject, for example a cloud or a rain cell, the radiometer antenna noisetemperature T_(A) (Az,El,t) rises compared to when the beam hits a clearsky area at the same azimuth or elevation value. The increase in theantenna noise temperature is bigger the more water is present in themain beam area when penetrating the atmosphere.

The results can, for example be presented as a plane diagram (FIG. 4)using the clear sky scanning antenna noise temperature curve

T_(A) ClearSky (Az,El) as reference level and by producing an outputfrom the scanned area of the noise temperature difference

T_(A) Diff (Az,El,t) observed by the radiometer antenna as a function oftime

    T.sub.A Diff (Az,El,t)=T.sub.A (Az,El,t)-T.sub.A ClearSky (Az,El).

The plane diagram changing by time provides antenna noise temperatureswhich deviate from the clear sky conditions, caused merely by water atthe microwave region 10 . . . 30 GHz. The deviations illustrate eitherrain S or cloud P, that is, weather changes in relation to clear sky.Sun is a strong noise source clearly visible in the radiometer antennanoise temperature even at beam widths over ten degrees. The time andplace for the presence of the sun are predictable and can if so desiredbe excluded from the output.

When the antenna main beam width is about one degree or more, the moon,the solar system planets, radio galaxies and other spatial radio sourcescause an insignificant change in the antenna noise temperature. Thecosmic background noise is independent of direction and does thus notaffect the relative values; it is contained in the basic reference.

Strong radio transmitters at the frequency used by the radiometer, andin visual communication with it, appear as dotted noise sources. Iftheir location is known, they can be excluded from the output.

Outdoor measurement provides a better knowledge of the ground brightnesstemperature thus enabling the calibration of the measuring results andthe clear sky reference level at an absolute scale. At big elevations(>30° C.) the rain-fall (mm/h) can be estimated based on knownproperties and the above mentioned calibration data. The effective pathlength of rain as function of elevation at different rain forces areknown. When the rain cell brightness temperature and the measurementelevation angle are known, an estimate can be calculated for the rainforce of the rain cell in question.

The radiometric scanners can be linked into operational networks (FIG.5a). When the locations of the scanners in relation to each other areknown, the objects causing atmospheric brightness temperature changescan be spatially delimited using the antenna noise temperature datagathered. With the output figure (FIG. 5b) the location, shape,structure and movement of the clouds and rain cells can be observed morein detail than with one scanner.

The radiometric scanner parameters, namely antenna noise temperature,medium frequency, bandwidth, integration time and sensitivity matchingare below presented using examples. The sensitivity of the radiometricreceiver has to be sufficient to observe weak objects, for example todistinguish a thin cloud at a short effective path, in the zenithdirection.

A cloud penetrated by the radiometer beam at the length of 1 km(effective path length) and which contains water 1 g/m³, causes abrightness temperature change of approx. 0,7 K at 12 GHz frequency whenreceived by the radiometer antenna beam. For the mentioned cloud tocause a measurable change in the radiometer system noise temperature,the sensitivity of the radiometer has to be 0,2 K or more. For example,with radio parameters such as: bandwidth 500 MHz, integration time 4 ms,amplification variation 5×10⁻⁴ and system noise temperature 200 K, asensitivity of 0,17 K is obtained.

The clear sky noise temperature of the antenna should also at smallelevation angles be considerably below the physical temperature of theatmospheric water (approx. 290 K), to provide a large scanner range. Theantenna noise temperature depends on the elevation angle, the radiometerfrequency and the antenna side lobe properties. At the absorptionresonance spectral peak frequency of the water molecule (22.3 GHz) andan elevation angle of 0° (FIG. 6a), the clear sky brightness temperatureis 290 K at an absolute humidity of 7,5 g/m³. As the brightnesstemperatures caused by clouds and rain cells are at maximum close to thephysical temperature (approx. 290 K) of rain, the change caused by therain in the sky brightness temperature cannot be observed at 22 GHzfrequency at 0°-elevation.

A rain cell with a diameter (effective path length) of 3 km and arainfall of 25 mm/h causes an increase in the clear sky brightnesstemperature of approx. 100 K at 10 GHz and approx. 190 K at 15 GHzfrequency. The observation of this from the horizon by its wholedynamics requires a sky brightness temperature of below 190 K (10 GHz)or below 100 K (15 GHz).

On the horizon the clear sky brightness temperature at 10 GHz is approx.130 K (<190 K) and at 15 GHz approx. 200 K (>100 K). Iteration giveswith a 100 K dynamic requirement a scanner upper frequency limit ofapprox. 12.5 GHz on the horizon.

Due to the earth curvature distant low objects stay beyond the horizon;objects visible on the horizon level from a distance of 100 km areapprox. 800 m above the ground. As water containing clouds and rainsoccur at a height of 0 . . . 4 km, part of the objects remains beyondthe horizon at distances over 100 km. A 4 km high rain cell at adistance of 100 km is visible at the 0°-1.8° elevation.

The sky brightness temperature at one degree elevation is severalKelvins lower than on the horizon. On the other hand, due to side lobeproperties the antenna noise temperature is several tens of Kelvinshigher than the sky brightness temperature at the same elevation,provided that the antenna main beam is fully above the horizon. Thedynamic requirements cause the facts presented above to cancel eachother out. Variation in the absolute humidity of air from `normal` 7.5g/m³ can change the sky brightness temperature several tens of Kelvins.This is seen as the dynamic area variation.

The inspection above provides an upper limit for the radiometric scannerfrequency of 12.5 GHz at a linear range of 100 km. The radiometerfrequency range should provide the biggest possible change area(dynamics) due to atmospheric water quantity changes. When moving fromthe scanner upper frequency limits to lower frequencies, the dynamicrange is restricted by the sensitivity relation of the radiometer to thesmall sky brightness temperature increase caused by weak objects. Thefrequency band used by the radiometric scanner should therefore be closeto the upper frequency limit, between 11-12.5 GHz.

The radiometer integration time has to be short in order to provide asufficient angular speed of the scanner to produce a dynamic brightnesstemperature picture. Assuming that the scanner produces an output of thefirmament four times an hour in order to obtain a realtime-picture, theelevation scanning speed has to be approx. 20°. . . 50° per second atthe beam width of two degrees, depending on the azimuth transitionbetween the elevation scannings. At a speed of 50° per second the beammoves two degrees in 40 ms. If the beam width is also in the range of 2°, the integration time must be considerably shorter than the transitiontime in the range of approx. 4 ms.

The antenna beam width should be as small as possible, in order toobtain a resolution and a small minimum elevation angle. The antennabeam width is inversely proportional to the diameter of the antenna. Thebigger antenna, the narrower beam, the better resolution and smallerminimum elevation angle (prior to the main beam hitting the ground). If,on the other hand, the firmament scanning is to be made with anarrow-beam antenna in the same time as with a wide-beam antenna, theangular speed has to be increased. The angular speed increase leads to ashortening of the integration time causing a weakening in the radiometersensitivity.

In order to achieve a quick picture update of the atmospheric cloud andrain cell changes and in relation to the scanner range to observe weakobjects with a sufficient sensitivity, a minimum limit-value of theantenna beam width of approx. 2° is obtained, which corresponds to anantenna diameter of 1 m at 12 GHz frequency.

The observation of a rain cell having a diameter of 3 km at a distanceof 100 km at full dynamics requires the filling of the antenna beam withthe object in question. This is realized at 12 GHz with an antennahaving a diameter of approx. 1 m.

The frequency band used by the radiometer should be free of strong radiotransmissions. The frequency band 11.00-12.75 GHz is generally reservedfor the downlink of geostationary satellites. These signals have a weakeffect and the locations of the transmitters can be identified.

Local weather condition, namely the occurrence of clouds and rain cells,their movements and changes can, for example, be observed with thedevice and method according to the invention (a radiometric scanner).The location of the clouds and the rain cells and a more accuratedefinition of their shapes is obtained with a network formed of severalscanners. This almost realtime and locally accurate weather conditioninformation can be utilized in addition to weather forecasts, forexample in agriculture, hydrology, pollution fallout location, traffic(air, land, sea) and in electromagnetic transmission attenuationforecasts (links and satellites).

One embodiment of the method according to the invention and the totaloutput radiometer adapted thereto is the rain detector. The heatingsystem 40 control of the satellite earth station antenna 41 is oneexample of the detector operation (FIG. 7).

A radiometer 12 furnished with a horn antenna is placed so that theradiometer antenna beam reflects from the earth station antenna 41reflector 42 to the sky. In clear sky conditions the radiometer antennanoise temperature is T_(Ant)(ClearSky).

If the radiometer antenna at the sky hits a cloud P or rain cell S or ifwet snow L has gathered on the antenna surface, these cause an increasein the radiometer antenna noise temperature T_(Ant)(Rain) in relation toclear sky condition. When the difference

    T.sub.Ant(Diff) =T.sub.Ant(Rain) -T.sub.Ant(ClearSky)

exceeds the set threshold value T_(Ant)(Threshold), the heating systemis controlled into operation. The method is for the satellite earthstation operation a direct measurement of the increase in the receivedsignal noise level and the attenuation of the receiving and transmissionproperties of the antenna caused by rain and snow gathered on theantenna surface. The radiometer beam width is in this application in thecategory of ten or tens of degrees. Due to fixed alignment and the slowchange rate of the observed phenomenon, the integration time can beseveral seconds. In order to be insensible to fouling and moisturegathered on the detector surface, its frequency should be low. On theother hand in order to have a sufficient sensitivity for observing waterthe frequency and band used should be high.

These mentioned conditions result in operational frequency limits of10-13 GHz.

The detector according to the invention can also be used so that theradiometer main beam sees the object giving the reference level directlyor via the conductive surface. The liquid substance or object moving toor gathered in the radiometer beam region causes the radiometric antennanoise temperature change.

What is claimed is:
 1. A method for indicating water content changethrough observing noise temperature changes, comprising the stepsof:receiving continuously electromagnetic microwave radiation with areceiver at a medium frequency below a lowest absorption resonance ofwater molecules; setting at least one noise temperature value as areference level; and comparing the noise temperature changes with thereference level.
 2. The method according to claim 1, further comprisingthe step of:generating output signals as responses to the receivedradiation at said medium frequency, said output signals representing thenoise temperature values of an antenna of the receiver at certainelevation and azimuth angles.
 3. The method according to claim 2,further comprising the step of:sweeping different elevation and azimuthangles with a virtual beam in a main axis of the antenna, the noisetemperature values changing with respect to the reference valuedetermined in clear sky conditions along with the moving of the antenna.4. The method according to claim 3, further comprising the stepsof:controlling the antenna with an antenna control unit, said controlunit receiving signals from a data processing/display unit whichreceives antenna signals from a radiometer; transmitting signals to saiddata processing/display unit; transmitting an azimuth control signal toan azimuth drive for controlling the antenna; receiving an azimuth anglesignal from an azimuth angle detector; transmitting an elevation controlsignal to an elevation drive for controlling the antenna; and receivingan elevation angle signal from an elevation angle detector.
 5. Themethod according to claim 4, further comprising the step of:forming anetwork of antennas and radiometers by positioning them spaced apart sothat same atmospheric regions are scanned by more than one antenna in atleast some areas.
 6. The method according to claim 1, further comprisingthe steps of:receiving a high frequency noise signal and mixing it witha local oscillator signal resulting in a lower, medium frequency noisesignal; limiting the bandwidth of the medium frequency noise signal bymeans of a band filter; amplifying, indicating and integrating themedium frequency noise signal to determine an antenna noise temperaturevalue of the radiometer at a microwave frequency region; comparing saidantenna noise temperature value with an antenna noise temperature valueobtained in clear sky conditions; and presenting weather informationderived from changes of the noise temperature value with respect to thereference level.
 7. The method according to claim 6, further comprisingthe step of:mixing the high frequency noise signal with the localoscillator signal resulting in medium frequency noise signal having afrequency of 10 to 15 GHz, with the antenna noise temperature valuebeing below 290 K at elevation angles smaller than 5°.
 8. The methodaccording to claim 7, wherein the intermediate frequency noise signalhas a frequency of 11 to 13 GHz.
 9. The method according to claim 8,wherein the medium frequency noise signal has a frequency of 11 to 12.75GHz.
 10. The method according to claim 5, further comprising the stepof:directing the antenna of the radiometer straight in a directionproviding the reference level for the noise temperature value.
 11. Themethod according to claim 5, further comprising the step of:directingthe antenna of the radiometer via a conducting surface in a directionproviding the reference level for the noise temperature value.
 12. Themethod according to claim 11, wherein the conductive surface is theantenna, comprising the further steps of connecting a heating device tothe antenna and heating the antenna based on a receipt of predeterminedcontrol signals from the radiometer.
 13. The method according to claim1, wherein the frequency is lower than 22 GHz.
 14. The method accordingto claim 13, wherein the frequency is considerably lower than 22 GHz.15. An apparatus for indicating water content change, comprising:anantenna configured to receive a noise signal; a band filter connected tothe antenna, the band filter limiting a bandwidth of the noise signal;an amplifier configured to amplify the bandlimited noise signal; adetector coupled to an output of the amplifier; a low pass filtercoupled to the detector and configured to filter the noise signalreceived from the detector; and an output providing a voltage which isproportional to the noise signal input to the detector.